The fluorinated amphoteric surfactants represented by the formulas below are described in greater detail in copending application of K. Mueller, Ser. No. 538,432, filed Jan. 3, 1975. When used as surfactants such compounds usually do not need to be isolated but are preferably used in a concentrated solution. The greater the solid content of such surfactants in a solution, the greater variety in final formulations (such as aqueous film forming foams) can be obtained. Said copending application discloses methods of preparation of concentrated solutions of such surfactants, but these methods produce solutions of at most only about 30% solids. The process of the instant invention is capable of producing solutions containing about 60% solids. Additionally, in known processes disclosed in Ser. No. 538,432, solid maleic anhydride is added to an aqueous solution of an amine which results in some hydrolysis to maleic acid which is an undesirable by product.
The hydrolysis of maleic anhydride can be eliminated by a process disclosed in Canadian Patent 828,195 which teaches the use of a nonaqueous solvent for the addition of maleic anhydride. The solvent has to be immiscible with water and not a solvent for the reaction product, and it must be removed before the second step (mercaptan addition) is carried out. This effectively minimizes diacid formation since the anhydride is dissolved, e.g. in methylene chloride, and is extracted from solution by the more reactive amine and not by water. However, in this process the solvent must be removed before a further reaction is carried out, which is tedious, time consuming and expensive which makes this approach commercially impractical. Furthermore, the process in said Canadian patent deals only with the first step of the reaction involved in the preparation of the surfactants and therefore in no way suggests how to prepare a solution having a higher solid content of the surfactant.